HEAT CAPACITY AND CALORIFIC INTENSITY CURVES 355
of air. This assumption is close enough for the purpose and saves
much laborious calculation. In addition it compensates to some
extent for the fact that the air naturally contains a certain amount
of water vapor and that all combustible gases carry water vapor,
being generally saturated at ordinary temperatures.
Table 6 gives the usual combustible elements of fuels, their
molecular weights and the amount of heat released in calories
per gram molecule at constant pressure, water assumed to remain a
vapor. While this last assumption gives lower thermal values it
agrees with practice, inasmuch as water vapor never condenses
within the furnace. The reaction formulas given in this table
do not contain the nitrogen. Table 7 is merely an extension of
this table, giving oxygen required for the hydrogen and carbon
and the total oxygen, the volume of nitrogen and the air volume,
as well as the volume of the products of combustion. This table
is based on the assumption that the air consists of 1 volume of
oxygen and 4 of nitrogen.
The first step in the plotting of. these curves is the computation
on Table 1, in which the volumetric composition of the gas is
considered as giving the number of gram molecules. These are
multiplied by the calories released per molecule at constant
pressure, and the summation of these values gives the total heat
released by the combustion of 100-g molecules of the gas. In
the case of solid or liquid fuels it is necessary to divide the weight
of each element multiplied by 10 by the molecular weight of the
substance or use weight values for the heat released. The prod-
ucts of combustion and the amount of oxygen required are tabu-
lated for the combustible portions of the fuel, and summed with
the inert portions of the fuel. When the fuel contains oxygen
the amount of this oxygen is deducted from the total of the
oxygen column and four times this value is deducted from the
total of the nitrogen column. The summation of these four
columns gives the total amount of oxygen required and the
products of complete combustion in air. If the weight of the
fuel is desired the number of molecules may be multiplied by the
molecular weights. This total weight may be readily converted
to the specific weight of a unit volume.
Should it be desired to note the effect of reducing the air
supply, deficiencies of 20 and 40 per cent are generally assumed
in order to get three points. Hydrocarbons are assumed to dis-